Add Xiaomi MiMo Responses providers

[foundev]

Summary

• Add first-class Xiaomi MiMo Responses API support for both plans
• Route Token Plan mimo::* models to token-plan-cn.xiaomimimo.com
• Route pay-as-you-go mimo-payg::* models to api.xiaomimimo.com
• Accept dedicated MIMO_TOKEN_PLAN_API_KEY and MIMO_PAY_AS_YOU_GO_API_KEY credentials
• Use Xiaomi's shared MIMO_API_KEY only when its documented tp- or sk- prefix identifies the corresponding plan
• Discover and enrich current MiMo model metadata, including reasoning levels, context size, and image support
• Preserve Xiaomi's documented sequential tool-call behavior
• Preserve provider pricing for pay-as-you-go while suppressing per-token pricing for subscription Token Plan usage
• Document credentials, endpoints, overrides, plan coexistence, and explicit model selection

Testing

• cargo test --workspace
• cargo test -p brokk-anvil-client mimo_client
• cargo clippy --workspace --all-targets -- -D warnings
• git diff --check